Method and system for determining a path between two points of an IP network over which datagrams are transmitted

ABSTRACT

System, computer program and method for determining a transmission path of datagrams in an IP network from a source device to a destination device. The source device receives from a network manager station, a message for retrieving information related to the transmission path of datagrams to the destination device. The message comprises a destination address and a source address. The destination address is an IP address of the destination device, and the source address is an IP address of the network manager device. The source device sends a plurality of probe datagrams to a respective plurality of IP network devices along the transmission path to the destination device. Each probe datagram comprises as destination address, the IP address of the destination device and as source address, the IP address of the network manager station. Consequently, replies, if any, to the probe datagrams are sent back directly to the network manager station by the IP network device along the transmission path, bypassing said the device. The invention also resides in the network manager station and the computer program product executing at the network manager station which sends the message to the source device, and receives and analyzes the replies to the probe datagrams to locate the break in the transmission path.

BACKGROUND OF THE INVENTION

The present invention relates generally to computer networks, and more particularly with a method, system and computer program for determining the transmission path of datagrams sent between two points of an Internet Protocol (IP) network.

Internet

The Internet is a global network of computers and computers networks which connects computers that use a variety of different operating systems or languages, including UNIX, DOS, Windows, Linux and Macintosh. To facilitate and allow the communication among these various systems and languages, the Internet uses a language referred to as TCP/IP (“Transmission Control Protocol/Internet Protocol”). TCP/IP protocol supports the following functions on the Internet:

-   -   transmitting and receiving electronic mail,     -   logging into remote computers (“Telnet”), and     -   transferring files and programs from one computer to another         (“FTP” or “File Transfer Protocol”).

TCP/IP

The TCP/IP protocol suite is named for two of the most important protocols:

-   -   a Transmission Control Protocol (TCP), and     -   an Internet Protocol (IP).         The first design goal of TCP/IP is to build an interconnection         of networks that provides universal communication services: an         “internetwork”, or “internet”. Each physical network has its own         technology dependent communication interface in the form of a         programming interface that provides basic communication         functions running between the physical network and the user         applications. The architecture of the physical networks is         hidden from the user. The second design goal of TCP/IP is to         interconnect different physical networks to form what appears to         the user to be one large network. TCP is a transport layer         protocol providing end to end data transfer. It is responsible         for providing a reliable exchange of information between two         computer systems. Multiple applications can be supported         simultaneously over one TCP connection between two computer         systems. IP is an internetwork layer protocol hiding the         physical network architecture below it. Part of the task of         communicating messages between computers is routing that ensures         that messages will be correctly directed within the network to         their destination. IP provides this routing function. An IP         message is called an IP Datagram. Application Level protocols         are used on top of TCP/IP to transfer user and application data         from one origin computer system to one destination computer         system. Such Application Level protocols are for instance File         Transfer Protocol (FTP), Telnet, Gopher, Hyper Text Transfer         Protocol (HTTP).

World Wide Web

With the increasing size and complexity of the Internet, tools have been developed to help find information on the network, often called navigators or navigation systems. Navigation systems that have been developed include standards such as Archie, Gopher and WAIS. The World Wide Web (“WWW” or “the Web”) is a recent superior navigation system. The Web is:

-   -   an Internet-based navigation system,     -   an information distribution and management system for the         Internet, and     -   a dynamic format for communicating on Internet.         The Web seamlessly integrates format of information, including         still images, text, audio and video. A user on the Web using a         graphical user interface may transparently communicate with         different host computers on the system, different system         applications (including FTP and Telnet), and different         information formats for files and documents including, for         example, text, sound and graphics.

IP Router

One of the basic functions of IP is its ability to form connections between different physical networks. This is due to the flexibility of IP to use almost any physical network below it, and to the IP routing algorithm. A system that builds connections between networks is termed a “router”. A “router” is a computer that interconnects two networks and forwards messages from one network to the other. Routers are able to select the best transmission path between networks. The basic routing function is implemented in the IP layer of the TCP/IP protocol stack, so any host (or computer) or workstation running TCP/IP over more than one interface could, in theory, forward messages between networks. Because IP implements the basic routing functions, the term “IP Router” is often used. However, dedicated network hardware devices called “routers” can provide more sophisticated routing functions than the minimum functions implemented in IP.

IP Routing

When data is sent to a remote destination, each IP datagram is first sent to a local router. An incoming datagram that specifies a destination IP address other than one of the local router IP address is treated as a normal outgoing datagram. This outgoing datagram is subject to the IP routing algorithm of the router, which selects the next hop for the datagram. The router forwards each datagram towards its final destination. A datagram travels from one router to another until it reaches a router connected to the destination. Each intermediate router along the end-to-end path selects the next hop used to reach the destination. The next hop represents the next router along the path to reach the destination. This next router can be located on any of the physical networks to which the intermediate router is attached. If it is a physical network other than the one on which the host originally received the datagram, then the result is that the intermediate router has forwarded the IP datagram from one physical network to another. An “IP routing table” in each router is used to forward datagrams between networks. A basic IP routing table comprises information about the locally attached networks and the IP addresses of other routers located on these networks, plus the networks to which they attach. A routing table can be extended with information on IP networks that are farther away, and can also comprise a default route, but it still remains a table with limited information. A routing table represents only a part of the whole IP networks. A router having such a routing table is called “a router with partial routing information”. A robust routing protocol must provide the ability to dynamically build and manage information in the IP routing table. As the changes in the network topology may occur, the routing tables must be updated with minimal or without manual intervention.

IP Path

When a datagram is sent to a remote destination, the succession of hops that the datagram flow through, form an “IP path”. The determination of the IP path is essential to understand how the datagram reaches its final destination when sent from an origin end point. Common network problems arise when the path is “broken”, that means when one (or more) intermediate router along the path is not able to properly forward datagrams to the next hop. To fix such a network problem, it is necessary, first, to isolate the problem and then, to identify the first hop that does not perform any more the routing function along the path.

Path Determination

The problem is therefore to find a way to determine the path that a datagram takes when this datagram is transmitted between two points of an IP network. The point of the network at the origin of the path to identify, can be called “source host”, and the point of the network at the end of this path, canl be called “destination host”. In many cases, routing problems occur between a client who tries to access resources located on a remote server. The object of the present invention is to locate network problems between a source and a destination host, by identifying the IP path between this source and destination hosts and determining where this path is broken. Usually, in corporate networks, users do not investigate problems by themselves, and even if they do so, they would not have sufficient authority and control over the network to fix the problem. In most of the cases, users call a help desk, and the investigation of problems is done by a person named “Network Manager”. This Network Manager is generally in a remote location and has enough authority to fix the network problem. The first task of the Network Manager is to isolate the problem and determine exactly where the path is broken.

Traceroute

A tool commonly used today in networks to troubleshoot connectivity problems, is a software application called “traceroute”. The traceroute application enables the determination of the route that datagrams follow from a source to a destination host. Traceroute is based upon UDP (User Datagram Protocol) and ICMP (Internet Control Message Protocol). Traceroute sends a series of “probe datagrams” using UDP to an IP address and waits for an ICMP reply. A first datagram with a “Time to Live” (TTL) of “one” is sent to the destination point. The first router to see the datagram decrements the TTL to zero and returns an ICMP “time exceeded” message. The datagram is discarded. In this way, the first router in the path is identified. This process is then repeated with successively larger TTL values in order to identify the series of routers in the path to the destination host. Each router through which the datagram passes similarly decrements the TTL by one, and when a router decrements the TTL to zero, it returns the ICMP message. If a datagram which is sent out does not elicit the expected ICMP reply, then the location of the break in the path can be determined by the TTL of this datagram, and the record of the other datagrams (with lower TTL values) for which the ICMP replies were received. Traceroute actually sends UDP datagrams to the destination point which reference a port number that is outside the normally used range. This enables traceroute to determine when the destination host has been reached, that is when an ICMP “port unreachable” message is received. In other terms, UDP datagrams are sent with monotonically increasing values in the “Time To Live” (TTL) field, and the chosen UDP port is one most likely not to be in use. For each TTL value, the traceroute program sends a fixed number of datagrams (usually three), and receives the IP addresses of the routers responding. This process continues until an ICMP “port unreachable” datagram is received or some TTL threshold is reached (usually thirty). If a router receives an IP datagram and decrements the TTL to zero, then it returns an ICMP “time exceeded” message. If the IP datagram eventually reaches the destination host, an ICMP “port unreachable” message is returned. Combining the information from all the replies the traceroute program can report the whole route.

Investigating network problems is a very common task devoted to Network Managers. A typical situation is when network equipment (for example, a customer's workstation) cannot reach another network equipment (for example, a server). The end user (e.g. the customer) will call a help desk, (typically the Network Manager) in charge of investigating and resolving the problem. The problem for the Network Manager is to perform the investigation of the failure remotely; usually the Network Manager is far away from the customer's workstation. Tivoli and the Netview console are typical tools for remotely investigating a problem on a network (Tivoli and Netview are trademarks of IBM Corporation). These tools give a view of the network, and of the status of the connections between neighbor network nodes. The availability of the ports on the network nodes are continuously monitored. A failure on a port will be immediately reported on the monitoring screen, and the Network Manager will be informed of the problem. However, a port failure represents only one type of problem among all the problems that can cause a connection failure. Other problems (such as routing table problems in routers) cannot be detected using this type of tools. This is why the traceroute application is helpful for investigating on a connection failure. By determining the actual path that is used by the datagrams in the network, and by indicating where this path is broken, traceroute allows a fast problem determination and failure isolation.

However, the usage of the traceroute application suffers from a major drawback. To execute a program such as traceroute on a computer or a system, the Network Manager needs to take control of the source host, either directly or remotely (remote logon). This implies the definition and usage by the Network Manager of passwords to take the control of the end user's workstation. The problem is that the end user (for example, the customer), may not want a third person to have access to his or her workstation for reason of confidentiality. The access by the Network Manager to a customer's workstation may be considered as an intrusion.

OBJECTS OF THE INVENTION

An object of the present invention is to enable a Network Manager to locate a break in the IP path between a source device and a destination device without taking control of the source device.

SUMMARY OF THE INVENTION

The present invention resides in a system, computer program and method for determining a transmission path of datagrams in an IP network from a source device to a destination device. The source device receives from a network manager station, a message for retrieving information related to the transmission path of datagrams to the destination device. The message comprises a destination address and a source address. The destination address is an IP address of the destination device, and the source address is an IP address of the network manager device. The source device sends a plurality of probe datagrams to a respective plurality of IP network devices along the transmission path to the destination device. Each probe datagram comprises as destination address, the IP address of the destination device and as source address, the IP address of the network manager station. Consequently, replies, if any, to the probe datagrams are sent back directly to the network manager station by the IP network device along the transmission path, bypassing said the device. The invention also resides in the network manager station and the computer program product executing at the network manager station which sends the message to the source device, and receives and analyzes the replies to the probe datagrams to locate the break in the transmission path.

In accordance with other features of the present invention, the plurality of probe datagrams include a “time to live” (TTL) value, said value being decremented by each IP network device along the transmission path. When an IP network device decrements the TTL value to zero, the IP network device replies to the network manager station.

BRIEF DESCRIPTIONS OF THE FIGURES

FIG. 1 shows a prior art technique used by Network Managers to monitor the availability of networks by testing the interfaces of network devices.

FIG. 2 shows a prior art technique used by Network managers to determine the path between two points of a network.

FIG. 3 shows a typical, prior art situation where a client device tries to connect to a server through a network having a failure on a connection.

FIG. 4 shows how a prior art traceroute program determines the path from a client towards a server and locates the first failure along this path.

FIG. 5 shows the structure of a prior art IP header layout of an IP datagram, said IP header comprising TTL and source address fields.

FIG. 6 shows how the path between the client and the server and the position of the first network failure along the path are determined and how the information is sent back to the Network Manager according to the present invention.

FIG. 7 is a flow chart showing the transfer of information between the different network devices (source and destination devices, routers along the IP path) and the Network Manager according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Path Determination Between a Client and a Server from a Network Manager

The present invention determines the transmission path of a datagram between two points in the network such as between a source host and a destination host or between a client station and a server. This determination is made from a point, such as a Network Management device or station, which is distant from the source host and client station. The present invention determines the transmission path without having to take control of the source host (client station) or manage remote logons (authorization, access lists, passwords, access control, etc . . . ). In the following description, the source, destination and Network Management devices are named “client station”, “server station” and “Network Manager station”, respectively. However, these names should not be interpreted as limiting, and in other embodiments of the invention, other types of devices may be used. The method for determining the transmission path of datagrams in a network and detecting a failure on this path according to the present invention comprises the following steps:

-   -   The Network Manager station sends a message (IP datagram) to the         client station (source host) to determine the path between this         client station and a server station (destination host). This         message comprises the IP address of the server station.     -   The client station receives the message sent by the Network         Manager station and retrieves from this message the following         information:         -   1. the IP address of the server station (destination host);         -   2. the IP address of the Network Manager station (source IP             address of the incoming message);     -   The client station then sends, using UDP (User Datagram         Protocol), a series of “probe datagrams” to the IP address of         the server station (destination host). IP datagrams carrying the         UDP datagrams are sent with monotonically increasing values in         the “time to live” (TTL) field of the IP header. For each TTL         value, a fixed number of datagrams are sent. However instead of         inserting in each outgoing datagram, its own source address         (client station IP address), the client station inserts the IP         address of the Network Manager station as source address. This         operation is called “spoofing”.     -   If a router receives one of the IP datagrams sent by the client         station and decrements the TTL to zero, then this router returns         an ICMP (Internet Control Message Protocol) “time exceeded”         datagram. The datagram returned is sent back according to the         source address of the datagram. In this case, the returned         datagram is directly sent back to the Network Manager station,         because of the spoofing technique described in the previous         paragraph.     -   If an IP datagram sent by the client station, eventually reaches         the server station (destination host), an ICMP “port         unreachable” datagram is returned to the Network Manager         station.         In the meantime the Network Manager station, combining the         information from all the replies, can report the whole route up         to the point where the first network failure occurs.

Network Monitoring Using Ping Application

There are two simple and widely used applications that are based on ICMP—Ping and Traceroute. Ping uses the ICMP Echo and Echo reply messages to determine whether a host is reachable. Ping is the simplest of all TCP/IP applications. It sends one or more IP datagrams to a specified destination host requesting a reply and measures the round trip time.

FIG. 1 shows a prior art method based on the Ping application to monitor, from a remote Network Manager station (103), the availability of a network connection between a client station (104) (source host) and a server station (105) (destination host). This method comprises the following steps:

-   -   From its work station, the Nework Manager (Help Desk) (103)         sends a probe IP datagram called “ping” (more properly named         “Echo Request”) towards each physical interface (port) of the         network devices located on the path that the Network Manager         wants to verify (101).     -   Upon reception of this datagram (if the interface is         operational), each network device sends back a reply (sometimes         called “Ping Response” but more properly named “Echo Reply”)         (102).     -   The Network Manager station waits for the network devices'         replies. A reply is received when the network device is         operational. In case of failure, no reply is received and the         Network Manager station displays a “timeout” message after a         given period of time.         By checking all individual interfaces along the path, the         Network Manager can know whether or not the interfaces are         working properly. However, this method has some draw backs. In         complex networks, the path taken by datagrams between a client         station and a server station is not always known. Many different         paths can be taken, and the selected path depends on complex IP         routing mechanisms. Furthermore, the path can be chosen         dynamically, if a dynamic routing protocol is used. Thus, it is         sometimes difficult to correlate an interface failure with a         connection problem, i.e. the interface failure can be unrelated         to the problem when for instance this failure is not on the         path. It is absolutely necessary to determine the path before         verifying whether or not the interfaces along said path are         working properly. In addition, a connection may be broken at a         logical level while all physical interfaces are operational. If,         for example, the routing function of one of the intermediate         network devices along the path is not properly working,         datagrams may be lost, although all interface are correctly         working. This is the reason why, it is very useful to have means         to determine the path that is actually used by the datagram         flow.

Determination of a Datagram Path with Traceroute

FIG. 2 Shows a prior art method based on the traceroute application used by Network Managers to determine the path used by a datagram sent by a client to a server. The Network Manager first takes control of the client station with a remote login (101). Then, the Network Manager launches in the client station, the traceroute program. As described earlier, the traceroute program sends probe datagrams along the path leading to the server station (209), causing each of the intermediate network devices (nodes, routers, . . . ) (210) to send back a reply ((202) to (206)). The client station (208) gathers the replies sent by the network devices, determines the entire path and transmits the information to the Network Manager (Help Desk) (207).

Failure on the Path Between Client and Server

FIG. 3 shows a typical, prior art situation where a client station (303) tries to reach a server station (304) without success due to a failure in the network. The path taken by the datagram is symbolized by the arrows (301). This path is interrupted at the point (302) where the network is broken. At this point, datagrams cannot be further transmitted.

Failure Investigation with Traceroute

FIG. 4 shows how the traceroute application operates to investigate a failure in a network according to prior art.

-   -   Probe datagrams are sent by the client station towards the         server station.     -   Intermediate network devices (nodes, routers, . . . ) reply to         the client station, up to the network failure (401 to 403)     -   The client station collects the replies and determines the         network path up to the network failure.         The last datagram sent by the client station, reaches the         network failure and is lost. No reply (404) is sent back to the         client station. After a predefined time period, the client         station presumes that a network failure is present just after         the path partially identified. The information is forwarded to         the Network manager which can easily and quickly locate the         failure.

Spoofing Technique

FIG. 5 shows the structure of an IP datagram header. The fields in the IP header mean the following:

-   -   VERS: This field identifies the IP protocol version.     -   HLEN: The length of the IP header counted in 32-bit quantities.         This length does not include the data field.     -   Service Type: The service type is an indication of the quality         of service (QoS) requested for the IP datagram.     -   Total Length: The total length of the datagram, header and data.     -   Identification: A unique number assigned by the sender to aid in         reassembling a fragmented datagram. Each fragment of a datagram         has the same identification number.     -   Flags: This field comprises control flags     -   Fragment Offset: This field is used to aid in reassembling the         full datagram. The value of this field is the number of 64-bit         segments (header bytes are not counted) that are contained in         earlier fragments. If this is the first (or only) fragment, this         field has a value of zero.     -   Time to Live (TTL): This field specifies the maximum time period         (in seconds) during which the datagram can travel.         Theoretically, each router processing this datagram is supposed         to subtract its processing time from this field. In practice, a         router processes the datagram in less than one second. Thus the         router subtracts one from the value indicated in this field. The         TTL has become a hop-count metric rather than a time metric.         When the value reaches zero, it is assumed that this datagram is         traveling in a closed loop and is discarded. The initial value         should be set by the higher level protocol that creates the         datagram.     -   Protocol Number: This field indicates the higher level protocol         to which IP should deliver the data in this datagram.     -   Header Checksum: This field is a checksum for the information         contained in the header. If the header checksum does not match         the contents, the datagram is discarded.     -   Source IP Address: The 32-bit IP address of the host sending         this datagram.     -   Destination IP Address: The 32-bit IP address of the destination         host for this datagram.         Among the IP header fields described here above, the source         address is different in the present invention. In the prior art,         this address is normally the IP address of the device that has         originated the datagram. This IP address is used by a number of         programs which need to know the origin of the datagrams. For         example, a program which needs to send a message of error back         to the originator of the datagram will use this IP address. This         is the case with the error message sent back when the time to         live field (TTL) reaches zero. This mechanism is used by the         traceroute application, in particular, to collect the replies to         its probe datagrams. However, in accordance with the present         invention, the “spoofing” technique consists of changing the         source IP address of the datagram to make the programs         processing the datagram think that the origin of the datagram is         different than what it is in reality. The error messages are         sent back to the new spoofed address and not to the real         originator (source host) of the datagram.

Method for Determining the Path and the Failure Along the Path According to the Present Invention

FIG. 6 shows how the path between the client station (610) and the server station (611) and the position of the first failure (609) along the path are determined. FIG. 6 also shows how the information is sent back to the Network Manager station (612) according to the present invention. The method of the present invention comprises the following steps:

-   -   A message (601) is sent from the Network Manager station (612)         by computer programming within the Network Manager station to         the client station (610) to launch the process.     -   Upon receipt of the message (601) sent by the Network Manager         station, the client station (including computer programming         executing within the client station) sends probe datagrams         toward the server station (611) with increasing TTL values         similarly to the traceroute program (602 to 604). These probe         datagrams (using the “spoofing” technique) include, as source         address, the IP address of the Network Manager station (instead         of the IP address of the client station).     -   The intermediate routers (613) send their replies back (when         TTL=0) according to the traceroute program described above.         However; because the intermediate routers rely on the spoofed         address inserted in the probe datagrams, the replies (606 to         608) are sent back directly to the Network Manager station.     -   The replies are collected by computer programming within the         Network Manager station, which determines the first part of the         path up to the network failure (or the complete path up to the         server station if there is no network failure).     -   The last probe datagram (605) sent by the client station (610)         reaches the network failure (609) and is lost. The Network         Manager station is then able to display the network path up to         the network failure (609).

FIG. 7 is a flow chart of the foregoing process showing the interaction between the various components of the network.

-   -   The Network Manager station (708) sends a command (701) to the         client station (709) to start the process and then waits for the         replies (702) sent back by routers (702).     -   The client station (709), upon receipt of the start command         (701), sends the probe datagrams (703 705 707 etc . . . ) with         as source address, the spoofed address of the Network Manager         station (708) and as destination address, the IP address of the         server station.     -   The intermediate routers (710) reply back directly to the         Network Manager station (704, 706, etc . . . ) when the TTL=0.     -   The replies (702) are collected by the Network Manager station.     -   A the end of the process, the Network Manager station (708)         displays (706) the path that has been computed based on the         replies sent by the routers (710).

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood that various changes in form and detail may be made therein without departing from the spirit, and scope of the invention. 

1. A method for determining a transmission path of datagrams in an IP network from a source device to a destination device, said method comprising the steps of: said source device receiving from a network manager station, a message for retrieving information related to the transmission path of datagrams to said destination device, said message comprising a destination address and a source address, said destination address being an IP address of the destination device, and said source address being an IP address of the network manager device; and said source device sending a plurality of probe datagrams to a respective plurality of IP network devices along the transmission path to the destination device, each of said probe datagrams comprising as destination address, the IP address of the destination device and as source address, the IP address of the network manager station, such that replies, if any, to said probe datagrams are sent back directly to the network manager station by the IP network device along the transmission path, bypassing said source device.
 2. The method according to claim 1 wherein the step of sending a plurality of probe datagrams to a respective plurality of IP network devices, comprises the step of: inserting a value in each said probe datagram sent to an IP network device to cause the respective IP network device to reply to said network manager station.
 3. The method according to claim 1 wherein the step of inserting a value in each probe datagram sent to a respective IP network device, comprises the step of: inserting said value in the “time to live” (TTL) field of the IP header of each said probe datagram, said value being decremented by each IP network device along the transmission path.
 4. The method according to claim 1 wherein said probe datagrams fully comply with a traceroute protocol except the source address in the IP header is replaced by the IP address of the network manager station.
 5. The method according to claim 1 wherein said IP network devices are IP routers.
 6. The method according to claim 1 wherein: said destination device is a server station; and said source device is a client station.
 7. A computer program product for determining a transmission path of datagrams in an IP network from a source device to a destination device, said computer program product comprising: a computer readable medium; first program instructions, executable at a network managing station, to send to said source device a message to retrieve information related to the transmission path of datagrams to said destination device by sending a plurality of probe diagrams to a respective plurality of IP network devices along the transmission path to the destination device, each of said probe datagrams comprising as destination address, an IP address of the destination device and as source address, an IP address of the network manager station, said message comprising a destination address and a source address, said destination address being the IP address of the destination device, and said source address being the IP address of the network manager device; and second program instructions, executable at said network managing station, to receive replies, if any, to said probe datagrams, said replies being sent back directly to the network manager station by the respective IP network devices along the transmission path, bypassing said source device; and wherein said first and second program instructions are recorded on said medium.
 8. The computer program product according to claim 7 wherein said plurality of probe datagrams comprises respective values in a field to cause the respective IP network device to reply to said network manager station.
 9. The computer program product according to claim 8 wherein the value in each probe datagram is a “time to live” (TTL) value in a TTL field, said value being decremented by each IP network device along the transmission path.
 10. The computer program product according to claim 7 wherein said probe datagrams fully comply with a traceroute protocol except the source address in the IP header is replaced by the IP address of the network manager station.
 11. The computer program product according to claim 7 wherein said IP network devices are IP routers.
 12. The computer program product according to claim 7 wherein: said destination device is a server station; and said source device is a client station.
 13. The computer program product according to claim 7 further comprising: third program instructions, executable in said network managing station, to determine a failure location in said transmission path based on said replies to respective probe datagrams, and absence of a reply to another one of said probe datagrams; and wherein said third program instructions are recorded on said medium.
 14. A computer system for determining a transmission path of datagrams in an IP network from a source device to a destination device, said computer system comprising: means for sending to said source device a message to retrieve information related to the transmission path of datagrams to said destination device by sending a plurality of probe datagrams to a respective plurality of IP network devices along the transmission path to the destination device, each of said probe datagrams comprising as destination address, an IP address of the destination device and as source address, an IP address of said computer system, said message comprising a destination address and a source address, said destination address being the IP address of the destination device, and said source address being the IP address of said computer system; and means for receiving replies, if any, to said probe datagrams, said replies being sent back directly to said computer system by the respective IP network devices along the transmission path, bypassing said source device; and means for determining a failure location in said transmission path based on said replies to respective said probe datagrams, and absence of a reply to another one of said probe datagrams.
 15. The computer system according to claim 14 wherein said plurality of probe datagrams comprise respective values in a field to cause the respective IP network device to reply to said computer system.
 16. The computer system according to claim 15 wherein the value in each probe datagram is a “time to live” (TTL) value in a TTL field, said value being decremented by each IP network device along the transmission path.
 17. The computer system according to claim 14 wherein said probe datagrams fully comply with a traceroute protocol except the source address in the IP header is replaced by the IP address of the computer system.
 18. The computer system according to claim 14 wherein said IP network devices are IP routers.
 19. The computer system according to claim 14 wherein: said destination device is a server station; and said source device is a client station. 